1. Field of the Invention
The present invention relates to a time-switched transmission diversity (TSTD) transmitter and a controlling method thereof in a code division multiple access (CDMA) mobile communication system, and more particularly, to a TSTD transmitter for limiting a power of a signal transmitted through any antenna to a maximum transmission power of the antenna and a controlling method thereof.
2. Description of the Related Art
In a mobile communication system, received signal levels relating to transmission of a radio frequency signal change rapidly. A rapid change in the receiving signal level is called fading. To reduce fading, a reception diversity scheme is used in which the number or arrangement of antennas in a mobile terminal is changed to improve reception sensitivity over that of a mobile terminal having a single antenna. Although the reception diversity scheme has improved reception performance, it has scarcely been used due of the required increased complexity of the mobile terminal and the resulting increase in cost.
Another option for reducing fading is using a third generation mobile communication system, such as an IMT-200 system, which attains the effect of reception diversity by using transmission diversity. In transmission diversity a plurality of antennas are installed in a base station instead of a mobile terminal. If the mobile station moves slowly (i.e., if the Doppler frequency is low) and if there are a small number of multiple paths (i.e., if there is little delay spread), the transmission diversity can obtain a diversity effect of up to 4-7 dB. Transmission diversity is typically classified into orthogonal transmission diversity (OTD), time-switched transmission diversity (TSTD), and selective transmission diversity (STD).
FIG. 1 illustrates a conventional OTD transmitter. Data is separated equally by a data separator 101 and then transmitted through first and second antennas via multiplexers (MUXs)102, serial/parallel (S/P) converters 103, mixers 104 and 105, complex PN (Pseudo Noise) spreaders 107, lowpass filters (LPFs) 109, mixers 110 and 111, and adders 112. An OTD transmitter, as shown in FIG. 1, requires an amplifier with half the power of a conventional power amplifier, since data is equally separated.
FIG. 2 illustrates a conventional TSTD transmitter. A MUX 113 converts logical signals xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d into actual transmission signals xe2x80x9c+1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d. An S/P converter 114 separately outputs even symbols and odd symbols. The even symbols are multiplied by an orthogonal code in a mixer 115 and then provided to a complex PN spreader 117, and the odd symbols are multiplied by the orthogonal code in a mixer 116 and then provided to the complex PN spreader 117. The complex PN spreader 117 complex-multiplies the outputs of the mixers 115 and 116 by I-channel and Q-channel data PN_I and PN_Q. A switching circuit 125 switches data which is separately output by the complex PN spreader 117 into data corresponding to the number of antennas to either one of two pairs of corresponding LPFs 119 and 120. A controller 121 controls the switching circuit 125 to switch user data in the unit of slots as shown in FIG. 3. The output of the LPF 119 is multiplied by a carrier frequency signal in a mixer 122 and then provided to an adder 124. The output of the LPF 120 is multiplied by a 90xc2x0 phase-shifted carrier frequency signal in a mixer 123 and then provided to the adder 124. The adder 124 adds the outputs of the mixers 122 and 123 to each other and transmits the added result via a corresponding antenna.
When data is switched in the unit of slots in the conventional TSTD scheme, a power Po assigned to the first antenna at a current period and a power Pe assigned to the second antenna at a current period are not equally distributed, as is illustrated in FIGS. 4B and 4C. FIG. 4A illustrates the transmission power for a NON-TSTD mode. The NON-TSTD mode illustrated in FIG. 4A needs one transmission amplifier, whereas the TSTD mode illustrated in FIGS. 4B and 4C requires, in the worst case, as many transmission amplifiers as the number of antennas.
As described above, a TSTD base station requires more transmission amplifiers than a non-TSTD base station and also requires the transmission amplifier to have a higher power than the transmission amplifier used in an OTD base station. This disadvantage regarding the TSTD base station results in increased cost and complexity when compared to non-TSTD base stations or OTD base stations.
In the TSTD mode, the power of a signal transmitted through any antenna may exceed a maximum transmission power of the antenna even if transmission amplifiers are provided with respect to every antenna, in which case, the signal may be lost.
It is, therefore, an object of the present invention to provide a device and method for limiting the power of a signal transmitted through any antenna to a maximum transmission power of the antenna in a TSTD transmitter.
It is another object of the present invention to provide a device and method for limiting a transmission power of an antenna by changing user data to a switching pattern of another antenna which does not exceed a maximum transmission power whenever the power of a signal transmitted through any antenna exceeds a maximum transmission power of that antenna.
In accordance with one aspect of the present invention, a device for limiting a transmission power of an antenna in a TSTD transmitter of a base station of a mobile communication system is provided, which comprises at least two antennas, a plurality of user channels, and a switching controller connected to the user channels, for switching user data received through the user channels to the antennas. The device includes a data state detector connected to the user channels, for measuring a power of user data on the user channels, a transmission power detector for measuring transmission powers of signals transmitted through the antennas, and an antenna pattern processor for storing a maximum transmission power of the antennas and switching pattern information for previously assigned user data, determining a switching pattern for assigning the new user data to a specific antenna of which transmission power added to a power of the new user data does not exceed the maximum transmission power, and controlling the switching controller and updating the switching pattern information, according to the determined switching pattern.
In accordance with another aspect of the present invention, a method for limiting a transmission power of an antenna in a TSTD transmitter of a base station of a mobile communication system is provided, the system comprising at least two antennas and a pattern storage for storing a maximum transmission power limit value and switching patterns of a plurality of user data to be transmitted through the antennas. The method comprises the steps of measuring transmission powers assigned to the respective antennas upon receipt of new user data, and determining a switching pattern such that a value obtained by adding one of the transmission powers to a power of the new user data does not exceed the maximum transmission power limit value.